This invention relates generally to Schmitt trigger circuits and more particularly to such circuits especially suited for CMOS (complementary metal-oxide-semicondoctor) integrated circuits.
A Schmitt trigger, also called a Schmitt limiter, is a bistable pulse generator circuit in which an output voltage undergoes a level transition with hysteresis in response to a changing input voltage. That is, the output voltage undergoes one level transition when the signal voltage rises to a high threshold voltage V.sub.th, and undergoes another, opposite level transition when the input signal voltage drops to a lower threshold voltage V.sub.tL. The difference between the two thresholds voltages (V.sub.th -V.sub.tL) is the "Schmitt width" and determines the amount of hysteresis of the trigger. Such triggers are used extensively wherever analog circuits interface with digital circuits. It is desirable that they be located with the digital circuit in the same integrated circuit. As a practical matter, this means that they are likely to be in CMOS technology.
One problem with present Schmitt trigger circuits is that typically the input threshold voltages and hysteresis are interdependent and are susceptible to changes in their values as a result of variations in the processing of the circuit in which they are incorporated. Means for compensating for such changes add considerably to the complexity of the circuit.